Windows of vehicles and aircraft typically have a relatively large surface area and, therefore, have good acoustic radiation efficiency. The resonance response of such window structures results in significant vibration and acoustic energy which typically contributes significantly to the interior noise in automobiles or other vehicles, and in aircraft cabins. Particularly for aircraft, turbulent air flow at the exterior surfaces of the fuselage skin and the exterior surfaces of the windows tends to excite these resonances.
In typical conventional aircraft, the windows have low-to-moderate inherent damping. An efficient countermeasure for the resonance response of a structure is to increase the effective damping, and in the case of windows, the optical quality of the window typically should be substantially maintained. It may be permissible to degrade the optical quality by a small amount, or perhaps for small surface areas, but in general the optical quality should not noticeably vary if damping is added to a window structure.
One conventional aircraft window is depicted in FIG. 1, generally designated by the reference numeral 10. This window structure 10 has an outer pane 12 that comprises stretched acrylic material, about 0.31 inches in thickness. A second pane 14 is also made of stretched acrylic material, and in this structure it is about 0.19 inches in thickness. Between the two panes 12 and 14 is an air gap, generally designated by the reference numeral 18. In this particular window structure the air gap is about 0.26 inches across, which is the distance that these two panes are spaced-apart from one another.
The edges of these two panes 12 and 14 are coated and sealed by silicone material, at 16. The window 10 is an example of a structure seen in some contemporary commercial passenger aircraft. In conventional windows as depicted in FIG. 1, the thicker pane 12 is the exterior pane for the window, while the thinner pane 14 faces the interior of the fuselage or cabin. In many of the conventional windows such as this, the interior pane 14 has a vent hole that tends to equalize the pressure on both sides of the pane 14.
A second type of aircraft window is depicted in FIG. 2, generally designated by the reference numeral 20. A first pane of material at 22 is made of thermal tempered glass, about 0.50 inches in thickness. This is the thicker pane, which could face (be in contact with) the environment to the exterior of the aircraft, or it could be used as the inner window structure, i.e., as the interior cabin pane. A second pane of thermal tempered glass is located at 24, and in this conventional window 20, the pane 24 is about 0.19 inches in thickness. Even though this pane 24 is less thick, it may be used as an exterior pane if desired, and it can be coated with a rain repellent coating on its outermost surface.
Instead of an air gap, a vinyl interlayer 26 separates the two glass panes 22 and 24. In this window 20, the vinyl interlayer is about 0.38 inches in thickness. There are also two urethane interlayers 28 that are about 0.02 inches in thickness. One of the urethane interlayers is between the outer pane 24 and the vinyl interlayer 26, while a second urethane interlayer is between the interior glass pane 22 and the vinyl interlayer 26. This window structure 20 also includes a conductive heating film 23 that is located on the inboard surface of the glass ply (or pane) 24.
The conventional window 20 includes other supporting structures that hold together the layers described above. A stainless steel Z-bar is positioned at 30, and tends to hold in place the outer glass pane 24 and the vinyl interlayer 26, in which a layer of sealant is disposed therebetween. The sealant layer is generally disposed at 32, and may comprise a type of sealant known as “PR 1425.”
An aluminum spacer member 40 is used as a support for the entire window structure 20. A silicone gasket 34 is placed on the outer surface of the Z-bar 30 and the aluminum spacer member 40. A metal insert 42 is placed within the vinyl interlayer 26, and makes contact with the aluminum spacer member 40. An edge filler structure made of phenolic is illustrated at 44, which tends to hold the inner pane 22 in place with the spacer member 40 and the vinyl interlayer 26.
Both of the conventional windows described above do not use any particular form of air-film vibration damping. The window structure 10 of FIG. 1 has a large air gap 18 which would not be particularly useful for air-film vibration damping, while the window structure 20 of FIG. 2 has no particular air gap at all.
It has been recognized by some that adding a viscoelastic material to a window may help reduce mechanical vibration or acoustical noise in moving vehicles. For example, the automotive industry is now including a clear viscoelastic material in window construction for some automobiles. The viscoelastic material in these automotive windows is located between two layers of glass or glass-like materials that would typically be found in an automotive windshield, for example. Moreover, the automotive viscoelastic materials cover the entire area of the panes of glass-like material. Of course, such viscoelastic material needs to be optically clear, so vehicle operators can see through such window structures to drive.
In these automotive industry applications, there are two panes of window-type material, in which the outer pane is the external pane that is in contact with the atmosphere, and the inner pane is in contact with the interior of the vehicle. These two panes are spaced-apart, and within this spaced-apart distance the panes are bonded to each other via a viscoelastic material which is optically “clear”. In other applications, viscoelastic materials are sometimes used between layers of metal for the purpose of vibration damping. In those situations, the optical “clarity” characteristics are not important.
It would be an improvement to add a form of vibration damping or acoustic radiation damping to the windows of vehicles and aircraft (or spacecraft for that matter). The higher the speed of the vehicle/aircraft/spacecraft, the more important that sufficient vibration damping of window structures may become.